14.0.6 14.8: There are trade-offs between growth, reproduction, and longevity.

If you were designing an organism, how would you structure its life history for maximum fitness? (Recall from Chapter 8 that fitness is an organism’s reproductive output relative to that of other individuals in the population.) Ideally, you would create an organism that (1) produces many offspring (beginning just after birth and continuing every year), while (2) growing tremendously large, to reduce predation, and (3) living forever.

Unfortunately, evolution operates with some constraints. Some of these traits are just not possible, because selection that changes one feature tends to adversely affect others. When evolution increases one life history characteristic, another characteristic is likely to decrease. Life history trade-offs can be better understood in the context of the three areas to which an organism allocates its resources: growth, reproduction, and survival. When resources are limited, increased allocation to one of these areas tends to reduce allocation to one or both of the others. Some examples of the best-studied trade-offs follow.

1. Reproduction and survival. Among big-bang reproducers such as the marsupial mouse (Antechinus) and salmon, investment in reproduction is exceedingly high, followed shortly by death (FIGURE 14-15). If individuals are physically prevented from reproducing, however, they can live many more years.

2. Reproduction and growth. Beech trees grow more slowly during years when they produce many seeds than in years when they produce no seeds. Similarly, female red deer kept from reproducing in their first year of life grow significantly larger during that year.

3. Number and size of offspring. The females of one lizard species, Uta stansburiana, can lay more eggs if the eggs are smaller, but a higher proportion of the offspring survive if the eggs are larger. Evolution has led to a medium-size egg that tends to maximize the number of surviving offspring.

There are many other trade-offs, such as the number of offspring produced and amount of parental care given. Additionally, among mammals, litter size increases with latitude (distance from the equator). This trend may be due to the trade-off between litter size and frequency. Closer to the equator, animals can breed for a longer portion of the year (year-round near the equator). Farther from the equator, the breeding season is shorter. To compensate for having fewer litters at higher latitudes, mammals produce more offspring per litter.

Can you think of other trade-offs?